1. Field of the Invention
The present invention relates to an LED drive circuit for driving an LED (light emitting diode), to a dimming device for dimming an LED, and to an LED illumination fixture, an LED illumination device, and an LED illumination system having an LED as a light source.
2. Description of Related Art
LEDs have such characteristics as low current consumption and long service life, and LED applications are expanding not only to display devices but to illumination fixtures and the like. In LED illumination fixtures, a plurality of LED units is often used in order to obtain the desired lighting intensity.
Common illumination fixtures usually use a commercial AC 100 V power supply, and in cases in which an LED illumination fixture is used in place of an incandescent bulb or other common illumination fixture, it is preferred that the LED illumination fixture also be configured to use a commercial AC 100 V power supply, the same as a common illumination fixture.
In the case of applying dimmer control to an incandescent bulb, a phase control dimmer (commonly referred to as an incandescent light control) is used in which dimmer control can easily be applied to the supply of power to the incandescent bulb by a single volume element, by switching on a switching element (usually a thyristor element or triac element) at a certain phase angle of an alternating-current power supply voltage.
FIG. 17 shows an example of the voltage of each component and the current waveform when an incandescent bulb 41 is operated by a phase control dimmer 2 (see FIG. 16). FIG. 17 shows the waveform of the output voltage V1 of an alternating-current power supply 1, the waveform of the voltage V41 across the terminals of the incandescent bulb 41, and the waveform of the current I41 which flows to the incandescent bulb 41. When a triac Tri1 switches from off to on, the voltage V41 across the terminals of the incandescent bulb 41 rapidly increases, the current I41 which flows to the incandescent bulb 41 also rapidly increases, and the incandescent bulb 41 is lit. Since current continues to flow to the incandescent bulb 41 thereafter as long as the triac Tri1 is on, the incandescent bulb 41 remains lit until the output voltage V1 of the alternating-current power supply 1 reaches a value near 0 V.
However, when the incandescent bulb 41 is dimmed by the phase control dimmer 2 as shown in FIG. 16, flickering or blinking is known to occur, and normal dimming is not possible when a low-wattage incandescent bulb is used as the incandescent bulb 41.
The same type of phase control dimmer used for dimmer control of an incandescent bulb is preferably used in cases in which dimmer control is applied to an LED illumination fixture that uses an alternating-current power supply. FIG. 18 shows an example of a conventional LED illumination system whereby dimmer control can be applied to an LED illumination fixture that uses an alternating-current power supply.
The LED illumination system shown in FIG. 18 is provided with a phase control dimmer 2, an LED module 3, and an LED drive circuit which has a diode bridge DB1, a switching control circuit CNT1, a switching element Q1, a coil L2, a diode D1, a capacitor C4, and a resistor R2. In the LED illumination system shown in FIG. 18, the switching control circuit CNT1 detects the value of the current flowing to the resistor R2 and the root-mean-square value of the voltage V2 that occurs at the positive output terminal of the diode bridge DB1, and controls the on/off state of the switching element Q1 on the basis of these detection results.
FIGS. 19A and 19B show examples of the waveform of the voltage V2 that occurs at the positive output terminal of the diode bridge DB1 in the LED illumination system shown in FIG. 18. FIG. 19A shows the waveform of the voltage V2 at a bright dimmer level, and FIG. 19B shows the waveform of the voltage V2 at a dark dimmer level. The voltage-current characteristic of the LED module 3 is shown in FIG. 20.
When the bright dimmer level is set, the triac Tri1 switches from off to on at a small phase angle (e.g., 45°), the voltage V2 occurring at the positive output terminal of the diode bridge DB1 rapidly rises (see FIG. 19A), and the LED module 3 is lit. Lighting of the LED module 3 is then maintained as long as the voltage across the terminals of the LED module 3 is above the forward voltage of the LED module 3. At this time, the LED module 3 is controlled at the operating point A in FIG. 20.
When the dark dimmer level is set, the triac Tri1 switches from off to on at a large phase angle (e.g., 141°), the voltage V2 occurring at the positive output terminal of the diode bridge DB1 rapidly rises (see FIG. 19B), and the LED module 3 is lit. Lighting of the LED module 3 is then maintained as long as the voltage across the terminals of the LED module 3 is above the forward voltage of the LED module 3. At this time, the LED module 3 is controlled at the operating point C in FIG. 20.
The current of the LED module 3 must be kept constant in order to maintain a constant brightness. However, small variations in the current value of the LED module 3 are unavoidable due to the temperature characteristics of such components as the LED module 3, the switching power supply unit for feeding current to the LED module 3, the phase control dimmer 2, and the alternating voltage outputted from the alternating-current power supply 1, and switch noise of the triac Tri1 of the phase control dimmer 2, switching noise of the switching power supply unit for feeding current to the LED module 3, and other noise.
Since the noise described above is present in the same amount regardless of the amount of current of the LED module 3, the amount of noise is relatively large when the current of the LED module 3 is small, and relatively small when the current of the LED module 3 is large. For example, a change of 1 mA in the current value of the LED module 3 due to the noise described above corresponds to an amount of current variation of about 1% at operating point A, and does not produce a noticeable light (luminance) variation, whereas at operating point C, such a change in the current value produces flickering of the LED, and a noticeable light (luminance) variation occurs.